This chapter provides technical details for various parts of the Yocto Project. Currently, topics include Yocto Project components and shared state (sstate) cache.

The BitBake task executor together with various types of configuration files form the Yocto Project core. This section overviews the BitBake task executor and the configuration files by describing what they are used for and how they interact. BitBake handles the parsing and execution of the data files. The data itself is of various types: Recipes: Provides details about particular pieces of software Class Data: An abstraction of common build information (e.g. how to build a Linux kernel). Configuration Data: Defines machine-specific settings, policy decisions, etc. Configuration data acts as the glue to bind everything together. For more information on data, see the "Yocto Project Terms" section in The Yocto Project Development Manual. BitBake knows how to combine multiple data sources together and refers to each data source as a layer. For information on layers, see the "Understanding and Creating Layers" section of the Yocto Project Development Manual. Following are some brief details on these core components. For more detailed information on these components see the "Reference: Directory Structure" appendix.

BitBake is the tool at the heart of the Yocto Project and is responsible for parsing the metadata, generating a list of tasks from it, and then executing those tasks. To see a list of the options BitBake supports, use the following help command: $ bitbake --help The most common usage for BitBake is bitbake <packagename>, where packagename is the name of the package you want to build (referred to as the "target" in this manual). The target often equates to the first part of a .bb filename. So, to run the matchbox-desktop_1.2.3.bb file, you might type the following: $ bitbake matchbox-desktop Several different versions of matchbox-desktop might exist. BitBake chooses the one selected by the distribution configuration. You can get more details about how BitBake chooses between different target versions and providers in the "Preferences and Providers" section. BitBake also tries to execute any dependent tasks first. So for example, before building matchbox-desktop, BitBake would build a cross compiler and eglibc if they had not already been built. This release of the Yocto Project does not support the glibc GNU version of the Unix standard C library. By default, the Yocto Project builds with eglibc. A useful BitBake option to consider is the -k or --continue option. This option instructs BitBake to try and continue processing the job as much as possible even after encountering an error. When an error occurs, the target that failed and those that depend on it cannot be remade. However, when you use this option other dependencies can still be processed.

The .bb files are usually referred to as "recipes." In general, a recipe contains information about a single piece of software. The information includes the location from which to download the source patches (if any are needed), which special configuration options to apply, how to compile the source files, and how to package the compiled output. The term "package" can also be used to describe recipes. However, since the same word is used for the packaged output from the Yocto Project (i.e. .ipk or .deb files), this document avoids using the term "package" when refering to recipes.

Class files (.bbclass) contain information that is useful to share between metadata files. An example is the Autotools class, which contains common settings for any application that Autotools uses. The "Reference: Classes" appendix provides details about common classes and how to use them.

The configuration files (.conf) define various configuration variables that govern the Yocto Project build process. These files fall into several areas that define machine configuration options, distribution configuration options, compiler tuning options, general common configuration options and user configuration options (local.conf, which is found in the Yocto Project files build directory).

By design, the Yocto Project build system builds everything from scratch unless BitBake can determine that parts don't need to be rebuilt. Fundamentally, building from scratch is attractive as it means all parts are built fresh and there is no possibility of stale data causing problems. When developers hit problems, they typically default back to building from scratch so they know the state of things from the start. Building an image from scratch is both an advantage and a disadvantage to the process. As mentioned in the previous paragraph, building from scratch ensures that everything is current and starts from a known state. However, building from scratch also takes much longer as it generally means rebuiding things that don't necessarily need rebuilt. The Yocto Project implements shared state code that supports incremental builds. The implementation of the shared state code answers the following questions that were fundamental roadblocks within the Yocto Project incremental build support system: What pieces of the system have changed and what pieces have not changed? How are changed pieces of software removed and replaced? How are pre-built components that don't need to be rebuilt from scratch used when they are available? For the first question, the build system detects changes in the "inputs" to a given task by creating a checksum (or signature) of the task's inputs. If the checksum changes, the system assumes the inputs have changed and the task needs to be rerun. For the second question, the shared state (sstate) code tracks which tasks add which output to the build process. This means the output from a given task can be removed, upgraded or otherwise manipulated. The third question is partly addressed by the solution for the second question assuming the build system can fetch the sstate objects from remote locations and install them if they are deemed to be valid. The rest of this section goes into detail about the overall incremental build architecture, the checksums (signatures), shared state, and some tips and tricks.

When determining what parts of the system need to be built, BitBake uses a per-task basis and does not use a per-recipe basis. You might wonder why using a per-task basis is preferred over a per-recipe basis. To help explain, consider having the IPK packaging backend enabled and then switching to DEB. In this case, do_install and do_package output are still valid. However, with a per-recipe approach, the build would not include the .deb files. Consequently, you would have to invalidate the whole build and rerun it. Rerunning everything is not the best situation. Also in this case, the core must be "taught" much about specific tasks. This methodology does not scale well and does not allow users to easily add new tasks in layers or as external recipes without touching the packaged-staging core.

The shared state code uses a checksum, which is a unique signature of a task's inputs, to determine if a task needs to be run again. Because it is a change in a task's inputs that triggers a rerun, the process needs to detect all the inputs to a given task. For shell tasks, this turns out to be fairly easy because the build process generates a "run" shell script for each task and it is possible to create a checksum that gives you a good idea of when the task's data changes. To complicate the problem, there are things that should not be included in the checksum. First, there is the actual specific build path of a given task - the WORKDIR. It does not matter if the working directory changes because it should not affect the output for target packages. Also, the build process has the objective of making native/cross packages relocatable. The checksum therefore needs to exclude WORKDIR. The simplistic approach for excluding the worknig directory is to set WORKDIR to some fixed value and create the checksum for the "run" script. Another problem results from the "run" scripts containing functions that might or might not get called. The incremental build solution contains code that figures out dependencies between shell functions. This code is used to prune the "run" scripts down to the minimum set, thereby alleviating this problem and making the "run" scripts much more readable as a bonus. So far we have solutions for shell scripts. What about python tasks? The same approach applies even though these tasks are more difficult. The process needs to figure out what variables a python function accesses and what functions it calls. Again, the incremental build solution contains code that first figures out the variable and function dependencies, and then creates a checksum for the data used as the input to the task. Like the WORKDIR case, situations exist where dependencies should be ignored. For these cases, you can instruct the build process to ignore a dependency by using a line like the following: PACKAGE_ARCHS[vardepsexclude] = "MACHINE" This example ensures that the PACKAGE_ARCHS variable does not depend on the value of MACHINE, even if it does reference it. Equally, there are cases where we need to add dependencies BitBake is not able to find. You can accomplish this by using a line like the following: PACKAGE_ARCHS[vardeps] = "MACHINE" This example explicitly adds the MACHINE variable as a dependency for PACKAGE_ARCHS. Consider a case with inline python, for example, where BitBake is not able to figure out dependencies. When running in debug mode (i.e. using -DDD), BitBake produces output when it discovers something for which it cannot figure out dependencies. The Yocto Project team has currently not managed to cover those dependencies in detail and is aware of the need to fix this situation. Thus far, this section has limited discussion to the direct inputs into a task. Information based on direct inputs is referred to as the "basehash" in the code. However, there is still the question of a task's indirect inputs, the things that were already built and present in the build directory. The checksum (or signature) for a particular task needs to add the hashes of all the tasks on which the particular task depends. Choosing which dependencies to add is a policy decision. However, the effect is to generate a master checksum that combines the basehash and the hashes of the task's dependencies. While figuring out the dependencies and creating these checksums is good, what does the Yocto Project build system do with the checksum information? The build system uses a signature handler that is responsible for processing the checksum information. By default, there is a dummy "noop" signature handler enabled in BitBake. This means that behaviour is unchanged from previous versions. OECore uses the "basic" signature handler through this setting in the bitbake.conf file: BB_SIGNATURE_HANDLER ?= "basic" Also within the BitBake configuration file, we can give BitBake some extra information to help it handle this information. The following statements effectively result in a list of global variable dependency excludes - variables never included in any checksum: BB_HASHBASE_WHITELIST ?= "TMPDIR FILE PATH PWD BB_TASKHASH BBPATH" BB_HASHBASE_WHITELIST += "DL_DIR SSTATE_DIR THISDIR FILESEXTRAPATHS" BB_HASHBASE_WHITELIST += "FILE_DIRNAME HOME LOGNAME SHELL TERM USER" BB_HASHBASE_WHITELIST += "FILESPATH USERNAME STAGING_DIR_HOST STAGING_DIR_TARGET" BB_HASHTASK_WHITELIST += "(.*-cross$|.*-native$|.*-cross-initial$| \ .*-cross-intermediate$|^virtual:native:.*|^virtual:nativesdk:.*)" This example is actually where WORKDIR is excluded since WORKDIR is constructed as a path within TMPDIR, which is on the whitelist. The BB_HASHTASK_WHITELIST covers dependent tasks and excludes certain kinds of tasks from the dependency chains. The effect of the previous example is to isolate the native, target, and cross-components. So, for example, toolchain changes do not force a rebuild of the whole system. The end result of the "basic" handler is to make some dependency and hash information available to the build. This includes: BB_BASEHASH_task-<taskname> - the base hashes for each task in the recipe BB_BASEHASH_<filename:taskname> - the base hashes for each dependent task BBHASHDEPS_<filename:taskname> - The task dependencies for each task BB_TASKHASH - the hash of the currently running task There is also a "basichash" BB_SIGNATURE_HANDLER, which is the same as the basic version but adds the task hash to the stamp files. This results in any metadata change that changes the task hash, automatically causing the task to be run again. This removes the need to bump PR values and changes to metadata automatically ripple across the build. Currently, this behavior is not the default behavior. However, it is likely that the Yocto Project team will go forward with this behavior in the future since all the functionality exists. The reason for the delay is the potential impact to the distribution feed creation as they need increasing PR fields and the Yocto Project currently lacks a mechanism to automate incrementing this field.

Checksums and dependencies, as discussed in the previous section, solve half the problem. The other part of the problem is being able to use checksum information during the build and being able to reuse or rebuild specific components. The shared state class (sstate.bbclass) is a relatively generic implementation of how to "capture" a snapshot of a given task. The idea is that the build process does not care about the source of a task's output. Output could be freshly built or it could be downloaded and unpacked from somewhere - the build process doesn't need to worry about its source. There are two types of output, one is just about creating a directory in WORKDIR. A good example is the output of either do_install or do_package. The other type of output occurs when a set of data is merged into a shared directory tree such as the sysroot. The Yocto Project team has tried to keep the details of the implementation hidden in sstate.bbclass. From a user's perspective, adding shared state wrapping to a task is as simple as this do_deploy example taken from do_deploy.bbclass: DEPLOYDIR = "${WORKDIR}/deploy-${PN}" SSTATETASKS += "do_deploy" do_deploy[sstate-name] = "deploy" do_deploy[sstate-inputdirs] = "${DEPLOYDIR}" do_deploy[sstate-outputdirs] = "${DEPLOY_DIR_IMAGE}" python do_deploy_setscene () { sstate_setscene(d) } addtask do_deploy_setscene In the example, we add some extra flags to the task, a name field ("deploy"), an input directory where the task sends data, and the output directory where the data from the task should eventually be copied. We also add a _setscene variant of the task and add the task name to the SSTATETASKS list. If you have a directory whose contents you need to preserve, you can do this with a line like the following: do_package[sstate-plaindirs] = "${PKGD} ${PKGDEST}" This method, as well as the following example, also works for mutliple directories. do_package[sstate-inputdirs] = "${PKGDESTWORK} ${SHLIBSWORKDIR}" do_package[sstate-outputdirs] = "${PKGDATA_DIR} ${SHLIBSDIR}" do_package[sstate-lockfile] = "${PACKAGELOCK}" These methods also include the ability to take a lockfile when manipulating shared state directory structures since some cases are sensitive to file additions or removals. Behind the scenes, the shared state code works by looking in SSTATE_DIR and SSTATE_MIRRORS for shared state files. Here is an example: SSTATE_MIRRORS ?= "\ file://.* http://someserver.tld/share/sstate/ \n \ file://.* file:///some/local/dir/sstate/" The shared state package validity can be detected just by looking at the filename since the filename contains the task checksum (or signature) as described earlier in this section. If a valid shared state package is found, the build process downloads it and uses it to accelerate the task. The build processes uses the *_setscene tasks for the task acceleration phase. BitBake goes through this phase before the main execution code and tries to accelerate any tasks for which it can find shared state packages. If a shared state package for a task is available, the shared state package is used. This means the task and any tasks on which it is dependent are not executed. As a real world example, the aim is when building an IPK-based image, only the do_package_write_ipk tasks would have their shared state packages fetched and extracted. Since the sysroot is not used, it would never get extracted. This is another reason why a task-based approach is preferred over a recipe-based approach, which would have to install the output from every task.

The code in the Yocto Project that supports incremental builds is not simple code. This section presents some tips and tricks that help you work around issues related to shared state code.

When things go wrong, debugging needs to be straightforward. Because of this, the Yocto Project team included strong debugging tools: Whenever a shared state package is written out, so is a corresponding .siginfo file. This practice results in a pickled python database of all the metadata that went into creating the hash for a given shared state package. If BitBake is run with the --dump-signatures (or -S) option, BitBake dumps out .siginfo files in the stamp directory for every task it would have executed instead of building the specified target package. There is a bitbake-diffsigs command that can process these .siginfo files. If one file is specified, it will dump out the dependency information in the file. If two files are specified, it will compare the two files and dump out the differences between the two. This allows the question of "What changed between X and Y?" to be answered easily.

The shared state code uses checksums and shared state memory cache to avoid unnecessarily rebuilding tasks. As with all schemes, this one has some drawbacks. It is possible that you could make implicit changes that are not factored into the checksum calculation, but do affect a task's output. A good example is perhaps when a tool changes its output. Let's say that the output of rpmdeps needed to change. The result of the change should be that all the "package", "package_write_rpm", and "package_deploy-rpm" shared state cache items would become invalid. But, because this is a change that is external to the code and therefore implicit, the associated shared state cache items do not become invalidated. In this case, the build process would use the cached items rather than running the task again. Obviously, these types of implicit changes can cause problems. To avoid these problems during the build, you need to understand the effects of any change you make. Note that any changes you make directly to a function automatically are factored into the checksum calculation and thus, will invalidate the associated area of sstate cache. You need to be aware of any implicit changes that are not obvious changes to the code and could affect the output of a given task. Once you are aware of such a change, you can take steps to invalidate the cache and force the task to run. The step to take is as simple as changing a function's comments in the source code. For example, to invalidate package shared state files, change the comment statments of do_package or the comments of one of the functions it calls. The change is purely cosmetic, but it causes the checksum to be recalculated and forces the task to be run again. For an example of a commit that makes a cosmetic change to invalidate a shared state, see this commit.

This section describes the mechanism by which the Yocto Project build system tracks changes to licensing text. The section also describes how to enable commercially licensed receipes, which by default are disabled.

The license of an upstream project might change in the future. In order to prevent these changes going unnoticed, the Yocto Project provides a LIC_FILES_CHKSUM variable to track changes to the license text. The checksums are validated at the end of the configure step, and if the checksums do not match, the build will fail.

The LIC_FILES_CHKSUM variable contains checksums of the license text in the source code for the recipe. Following is an example of how to specify LIC_FILES_CHKSUM: LIC_FILES_CHKSUM = "file://COPYING;md5=xxxx \ file://licfile1.txt;beginline=5;endline=29;md5=yyyy \ file://licfile2.txt;endline=50;md5=zzzz \ ..." The Yocto Project uses the S variable as the default directory used when searching files listed in LIC_FILES_CHKSUM. The previous example employs the default directory. You can also use relative paths as shown in the following example: LIC_FILES_CHKSUM = "file://src/ls.c;startline=5;endline=16;\ md5=bb14ed3c4cda583abc85401304b5cd4e" LIC_FILES_CHKSUM = "file://../license.html;md5=5c94767cedb5d6987c902ac850ded2c6" In this example, the first line locates a file in S/src/ls.c. The second line refers to a file in WORKDIR, which is the parent of S. Note that this variable is mandatory for all recipes, unless the LICENSE variable is set to "CLOSED".

As mentioned in the previous section, the LIC_FILES_CHKSUM variable lists all the important files that contain the license text for the source code. It is possible to specify a checksum for an entire file, or a specific section of a file (specified by beginning and ending line numbers with the "beginline" and "endline" parameters, respectively). The latter is useful for source files with a license notice header, README documents, and so forth. If you do not use the "beginline" parameter, then it is assumed that the text begins on the first line of the file. Similarly, if you do not use the "endline" parameter, it is assumed that the license text ends with the last line of the file. The "md5" parameter stores the md5 checksum of the license text. If the license text changes in any way as compared to this parameter then a mismatch occurs. This mismatch triggers a build failure and notifies the developer. Notification allows the developer to review and address the license text changes. Also note that if a mismatch occurs during the build, the correct md5 checksum is placed in the build log and can be easily copied to the recipe. There is no limit to how many files you can specify using the LIC_FILES_CHKSUM variable. Generally, however, every project requires a few specifications for license tracking. Many projects have a "COPYING" file that stores the license information for all the source code files. This practice allows you to just track the "COPYING" file as long as it is kept up to date. If you specify an empty or invalid "md5" parameter, BitBake returns an md5 mis-match error and displays the correct "md5" parameter value during the build. The correct parameter is also captured in the build log. If the whole file contains only license text, you do not need to use the "beginline" and "endline" parameters.

By default, the Yocto Project build system disables components that have commercial licensing requirements. The following four statements in the $HOME/poky/meta/conf/distro/poky.conf file disable components: COMMERCIAL_LICENSE ?= "lame gst-fluendo-mp3 libmad mpeg2dec ffmpeg qmmp" COMMERCIAL_AUDIO_PLUGINS ?= "" COMMERCIAL_VIDEO_PLUGINS ?= "" COMMERCIAL_QT ?= "qmmp" If you want to enable these components, you can do so by making sure you have the following statements in the configuration file: COMMERCIAL_AUDIO_PLUGINS = "gst-plugins-ugly-mad \ gst-plugins-ugly-mpegaudioparse" COMMERCIAL_VIDEO_PLUGINS = "gst-plugins-ugly-mpeg2dec \ gst-plugins-ugly-mpegstream gst-plugins-bad-mpegvideoparse" COMMERCIAL_LICENSE = "" COMMERCIAL_QT = "" Excluding a package name from the COMMERCIAL_LICENSE or COMMERCIAL_QT statement enables that package. Specifying audio and video plug-ins as part of the COMMERCIAL_AUDIO_PLUGINS and COMMERCIAL_VIDEO_PLUGINS statements includes the plug-ins into built images - thus adding support for media formats.